Looking more flora than fauna, a plate-size "solar powered" nudibranch—Phyllodesmium longicirrum—farms zooxanthellae algae within its own body. Feeding on the soft coral Sarcophyton, the nudibranch pilfers algal cells and hoards them within its digestive system, which fills the paddle-shaped appendages called cerata. Stored just beneath the skin, the algae capture light energy, producing nutrients that can sustain the nudibranch for months. The same chemicals that feed the animal are also exuded from its skin as a defensive shield.

Built to feed exclusively on corals like this spindly gorgonian, a translucent 1.7-inch-long Phyllodesmium iriomotense houses its branching digestive gland within tentacle-like cerata—outgrowths the animal can shed if under attack. This species is one of the few colorless nudibranchs.

As adults, nudibranchs can be finicky eaters: A zebra-striped species of Armina, a genus that ranges to 8 inches long, plays tug-of-war with its sole prey, the burrowing sea pen. A tiny blenny bystander picks the safer perch.

Inch-long Hypselodoris whitei mate head to tail through a genital aperture. Equipped with both male and female reproductive organs—as are all nudibranchs—each fertilizes the other, and both produce eggs.

Common hitchhiker, an imperial shrimp (Periclimenes imperator) feeds—perhaps on sea debris or fecal pellets—where its host, a Ceratosoma nudibranch, has laid a ribbon of eggs. The shrimp benefits from the relationship, gaining a ride and access to food, but scientists don't know whether the slug profits from the shrimp's cleanup behavior.

With a flip of its skirt, Hexabranchus sanguineus can flee danger by taking to the open sea—a skill many of its cousins lack. Known as the Spanish dancer, the species is a giant among nudibranchs; some grow to a foot and a half, on a diet strictly of sponges. When not flashing its contrasting mantle colors to distract predators, the animal is actually camouflaged in its habitat.

Also able to camouflage themselves are tiny imperial shrimp, Periclimenes imperator, which at 0.7 inch long can nestle in the gills of a Spanish dancer, finding safe harbor and bits of food. Whether the tag-along shrimp benefit their host is uncertain—one of many nudibranch puzzles yet to be solved.

A Ceratosoma nudibranch appears to attack a stock-still variable sabretooth blenny (Petroscirtes variabilis). But this was not a bite (the nudibranch eats sponges), just happenstance in the night. Sea slugs crawl over whatever lies in their path, and many bottom-dwelling fish tolerate being crossed.

With sensory rhinophores like bulging eyes, a sidegill slug—Euselenops luniceps—swims freely by flapping its "wings" through dark seas. Though built with the tools to travel in open water, this animal, a close relative of the nudibranch, spends most of its life burrowed in the sand.

A quartet of Risbecia tryoni nudibranchs show the beginnings of trailing behavior, in which the animals follow one another's slime trails, each hot on the tail of the next. (It often occurs in pairs.) Scientists once thought trailing was related to mating, but evidence is thin; its true purpose remains unknown.